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This article explores myths about the scientific method, comparing classical experimental science with historical science. It discusses the acquisition of evidence through fieldwork and the search for a "smoking gun" to justify historical hypotheses.
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Common Cause Explanation & The Search for a “Smoking Gun” Carol E. Cleland Philosophy Department Center for Astrobiology University of Colorado (Boulder)
OVERVIEW • Myths about the scientific Method. • Classical experimental science and prototypical historical science: two different but equally rational and objective patterns of evidential reasoning. • How evidence acquired through field work justifies historical hypotheses: Common cause explanation and the search for a “smoking gun”.
Myths about the Scientific method • Inductivism: Scientists prove theories and hypotheses by a logical process of induction.
Myths about the Scientific method Falsificationism: Scientists falsify theories and hypotheses by using empirical evidence to refute them.
The Logic of Prediction Basic Concepts “Toy” Example All copper expands when heated. If sample of copper #4 is heated, then it will expand (Heating copper sample #4) (Copper sample #4 will expand) 1. Hypothesis(H): (All C’s are E’s) 2. Test Implication (I): (If x is a C, then x is an E.) • Test Condition (C): 4. Prediction (E):
The Logic of Evaluating the Results of an Experiment • Successful Prediction 1. If H, then I 2. I C. H • Logical Fallacy: “affirming the consequent”. (This is just another version of the problem of induction.)
The Logic of Evaluating the Results of an Experiment • Failed Prediction 1. If H, then I 2. Not-I C. Not-H • Valid Argument Form: “denying the consequent”. (This explains the appeal of falsificationism.)
The Terrible Truth about Falsificationism The form of the first premise in the previous argument is: If H and A, then I (where ‘A’ stands for a set of auxiliary assumptions {a1, a2, …, an} about other conditions, known and unknown, about the actual experimental situation.)
The Terrible Truth about Falsificationism (continued) • This changes the form of the argument to: 1. If H and {a1, a2, …, an}, then I 2. Not-I 3. Not-(H and {a1, a2, …, an}) 4. Not-H or not-{a1, a2, …, an} 5. Not-H or not-a1 or not-a2 or … or not-an (by De Morgan’s theorem)
The Terrible Truth about Falsificationism (continued) From a logical standpoint, no observation (whether experimental or in the field), can conclusively falsify a hypothesis. For it is always possible to salvage the hypothesis in the face of a failed prediction by denying an auxiliary assumption.
More Nails in the Coffin of falsificationism • Falsificationism is not only logically but also historically flawed. • Faced with failed predictions scientists have historically denied auxiliary assumptions, e.g., perturbations in the orbits of Uranus & Mercury. • Falsificationism is inconsistent with the practice of scientists & training of young scientists • Faced with failed predictions scientists typically deny and indeed are trained to deny auxiliary assumptions rather than the target hypothesis.
Conclusion Neither inductivism nor falsificationism provides a satisfactory account of any scientific practice; the scientific method of yore is a myth.
Part II Differences in the methodology of classical experimental science and prototypical historical, natural science: Is historical natural science methodologically inferior to experimental science?
The structure of Classical Experimental Science • Focus:Is on a single (sometimes complex) hypothesis which typically has the form of a universal generalization (All C’s are E’s). • Central Research Activity:Consists in repeatedly bringing about the test conditions specified by the hypothesis and controlling for extraneous conditions that might be responsible for false positives and false negatives.
The Experimental Program vs. Solitary Experiment • Failed predictions: do not result in the rejection of hypotheses; they are best interpreted as attempts to protect the hypothesis from false negatives. • Successful predictions: Are not followed by risky tests (in Popper’s sense); they are best interpreted as attempts to protect the hypothesis from false positives. • Acceptance/rejection of a hypothesis: occurs only after a hypothesis is subjected to a series of experiments controlling for plausible auxiliary assumptions that could explain predictive successes and predictive failures.
The structure of Prototypical Historical Science Focus: Is on proliferating multiple, rival hypotheses to explain a puzzling body of traces of past events (data) encountered in field work. Central Research Activity: Consists in searching for a ‘smoking gun’ a trace(s) that sets apart one or more hypotheses as providing a better explanation for the body of traces thus far acquired than the others.
A Case StudyThe Alvarez Hypothesis • Two-pronged hypothesis: 1) impact; 2) extinction. • Initially many different explanations for the end-Cretaceous mass extinction: pandemic, evolutionary senescence, climate change, supernova, volcanism, and meteorite Impact. • Discovery of an iridium anomaly (“smoking gun”) in K-T boundary sediments narrowed it down to two possibilities: volcanism and meteorite impact. Discovery of extensive quantities of a rare form of shocked mineral subsequently cinched the case for impact over volcanism.
A Case Study: The Alvarez Hypothesis (cont) • Paleontologists weren’t convinced: They agreed that there had been a meteorite impact but many doubted that it caused the end-Cretaceous extinctions. • The discovery of extensive pertinent fossil evidence (especially small organisms such as foraminifera and ammonites, and fern spores and angiosperm pollin) on either side of the K-T boundary was pivotal in changing their minds, providing the needed smoking gun for the second prong (mass extinction) of the hypothesis.
Lessons from the Alvarez hypothesis: The evaluation of historical hyotheses is: • Not grounded in prediction: • Historical predictions are not ‘risky’ in Popper’s sense; too many highly plausible extraneous conditions (e.g., iridium poor meteorite, geological processes of concentration and dispersal, unrepresentative samples of K-T boundary) capable of defeating them. • Predictions are typically vague, e.g., Ward’s ‘prediction’ about Cretaceous ammonites; they serve more as roadmaps for looking for a smoking gun than predictions.
The Evaluation of Historical Hypotheses (cont.) • A hypothesis may be rejected on the basis of evidence that does not refute it, e.g., the contagion hypothesis for the end-Cretaceous extinctions. • The acceptance of a hypothesis does not require a successful prediction, e.g., the iridium anomaly was not and could not have been predicted or retrodicted.
The Evaluation of Historical Hypotheses(cont.) • Grounded in explanatory power: • Hypotheses are accepted and rejected in virtue of their power to explain (vs. predict) puzzling bodies of traces discovered through field work. • The Alvarez hypothesis explains an otherwise puzzling association (correlation) among traces better than any of its rivals. It is for this reason that it is viewed as ‘confirmed’ and its rivals are no longer seriously entertained by scientists.
Part III Common cause explanation and the search for a smoking gun
Common Cause explanation • Reichenbach’sPrinciple of the Common Cause: seemingly improbable associations (correlations or similarities) among traces are best explained by reference to a common cause. C • Presupposes that the temporal structure of causal relations in our universe is such that most (not all) events form causal forks opening from past to future (leave many traces in the future). E1 E2 E3 E4
Common cause explanation (cont.): But is there any reason to believe the principle of the common cause is true?
YES!The Asymmetry of Overdetermination • A time asymmetry of causation: Most localevents & structures overdetermine their past causes (because the latter typically leave extensive and diverse effects)andunderdetermine their future effects (because they rarely constitute the total cause of an effect) • Much easier to infer an ancient volcanic eruption than a near future volcanic eruption.
The Asymmetry of Overdetermination (cont.) • Physical source is controversial but it characterizes all wave (radiative asymmetry)and particle (2nd law of thermodynamics) phenomena above the quantum level; an objective and pervasivephysical feature of world. • Physically (vs. logically or strictly metaphysically) grounds the Principle of the Common Cause and the methodology of historical natural science: the Search for a smoking gun.
An illustration: The colors of dinosaurs Asym of OD Asserts that the present is filled with overdetermining traces of the past; hence one can never completely rule out finding a smoking gun for any scientific hypothesis about the past. The methodology of historical field work is based upon this possibility.
Conclusions • Historical Scientists exploit the overdetermination of the past by the localized present by searching for a smoking gun to discriminate among competing hypotheses; the asymmetry of overdetermination guarantees there are likely to be many such telling traces. The problem is recognizing them for what they represent.
Conclusions 2. Experimental scientists try to circumvent the underdetermination of the future by the localized present by constantly testing for false positives and false negatives that might yield misleading confirmations or disconfirmations of their hypotheses; the asymmetry of overdetermination guarantees that this is always a threat. • There are no records of the future.
Conclusions 3. The methodology of historical science is different from that of classical experimental science but it is not inferior; each practice is designed to exploit the differing information that nature puts at its disposal.
References • “Common cause explanation and the search for a smoking gun” in Baker, V. (ed.), 125th Anniversary volume of the Geological Society of America (forthcoming). • “Prediction and Explanation in Historical Natural Science,”British Journal of Philosophy of Science 62 (2011), 551-582. • “Philosophical issues in natural history and its historiography” in Tucker, A. (ed.), Blackwell Companions to Philosophy: A Companion to the Philosophy of History and Historiography. Oxford: Blackwell Pub. (2009), pp. 44-62. • “Methodological and Epistemic Differences Between Historical Science and Experimental Science,”Philosophy of Science 69, (2002), pp. 474-496. • “Historical science, experimental science, and the scientific method,”Geology 29, (2001), pp. 987-990.
